1. Field of the Invention
The present invention relates to a high frequency ceramic compact for use in the microwave region, millimeter wave region, and other high frequency regions, and to a method of producing the same. Additionally, the present invention relates to a dielectric antenna, support for dielectric resonator, dielectric resonator, dielectric filer, dielectric duplexer and communication system, which are mounted on, for example, mobile phones, personal radios, satellite receivers, local area wireless networks and millimeter wave radars.
2. Description of the Related Art
Ceramic compacts are conventionally widely used in, for example, dielectric resonators and circuit boards for use in the microwave region, millimeter wave region and other high frequency regions.
Such high frequency ceramic compacts must have (1) a low dielectric loss, that is, a high Q-value, and (2) a thermally stable resonant frequency, that is, a temperature coefficient of resonant frequency (xcfx84f) that can be optionally controlled in the vicinity of 0 ppm/xc2x0 C.
In order to miniaturize the resulting device, a high relative dielectric constant (∈r) of the ceramic is preferable since the wavelength of an electromagnetic wave in the dielectric is shortened to 1/(∈r)1/2. However, demands have also been made to provide a material having a low relative dielectric constant, as the dielectric device becomes excessively small in size to deteriorate workability when the device is used at high frequencies.
This type of conventional high frequency ceramic compacts includes, for example, a Ba(Sn,Mg,Ta)03 based ceramic as described in Japanese Examined Patent Application Publication No. 3-34164, and a MgOxe2x80x94SiO2xe2x80x94Al2O3 based ceramic as described in Japanese Examined Patent Application Publication No. 6-103603 and Japanese Unexamined Patent Application Publication No. 8-69715.
The Ba(Sn,Mg,Ta)03 based ceramic compact can control the temperature coefficient of resonant frequency (xcfx84f) in the vicinity of 0 ppm/xc2x0 C. and has a high Q-value at 10 GHz from 20,000 to 30,000. However, the ceramic compact has a high relative dielectric constant (∈r) of 24 and the resulting device is excessively small in size to thereby deteriorate workability when it is used in a microwave region or millimeter wave region.
In contrast, the MgOxe2x80x94SiO2xe2x80x94Al2O3 based ceramic compacts and alumina ceramics have a low relative dielectric constant (∈r) from 7 to 10 and a high Q-value at 10 GHz from 6,000 to 29,000. These ceramics, however, have a high absolute value of the temperature coefficient of resonant frequency (xcfx84f) from xe2x88x9230 to xe2x88x9250 ppm/xc2x0 C. and are limited in application as dielectric materials for use in a microwave region or millimeter wave region.
A possible solution to control the temperature coefficient of resonant frequency (xcfx84f) of the alumina ceramic compacts is a combination use with TiO2 which has a positive temperature coefficient of resonant frequency (xcfx84f). This type of raw material, however, must be sintered by firing at temperatures not lower than 1350xc2x0 C., and firing at such high temperatures invites the formation of an Al2TiO5 crystal phase to thereby deteriorate the Q-value and other characteristics.
Accordingly, an object of the present invention is to solve the above problems and to provide a high frequency ceramic compact that is excellent in high frequency characteristics and temperature characteristics, has a relative dielectric constant (∈r) of about 20 or less and a Q-value at 10 GHz of about 10,000 or more and can optionally control the temperature coefficient of resonant frequency (xcfx84f) around 0 ppm/xc2x0 C., and to provide a method of producing the high frequency ceramic.
Another object of the present invention is to provide, using the aforementioned ceramic compact, a dielectric antenna, support for dielectric resonator, dielectric resonator, dielectric filter or dielectric duplexer, each of which is excellent in electric characteristics, and a high-performance communication system of small size.
Specifically, the present invention provides, in an aspect, a high frequency ceramic compact which includes Al, Ti and Mn as metallic elements and contains substantially no Al2TiO5 crystal phase.
The high frequency ceramic compact is preferably obtained by firing at a temperature not exceeding about 1310xc2x0 C.
In another aspect, the present invention provides a high frequency ceramic compact which has a Q-value at 10 GHz of 10000 or more and includes Al, Ti and Mn as metallic elements, is represented by the following formula: (100xe2x88x92xxe2x88x92y)AlO3/2 xe2x88x92xTiO2xe2x88x92yMnO, wherein x and y are % by mole and x and y satisfy the following conditions: 3.0xe2x89xa6xxe2x89xa69.0; and 0.1xe2x89xa6yxe2x89xa61.0.
Specifically, x and y in the compositional formula preferably further satisfy the following conditions: 3.0xe2x89xa6xxe2x89xa67.0 and 0.1 less than yxe2x89xa60.25. Preferably, substantially no Al2TiO5 crystal phase is contained in the high frequency ceramic.
The compositional formula is the compositional formula of the high frequency ceramic compact after sintering. As raw materials for the high frequency ceramic compact, an alumina powder having a specific surface area of about 4 m2/g or more and a titanium dioxide powder having a specific surface area of about 3 m2/g or more are preferably used as the raw material of AlO3/2 component and the raw material of TiO2 component, respectively. An alumina powder having a specific surface area from about 4 m2/g to 5 m2/g is particularly preferably used as the raw material of AlO3/2 component.
The present invention provides, in a further aspect, a method of producing a high frequency ceramic compact represented by the following formula: (100xe2x88x92xxe2x88x92y)AlO3/2xe2x88x92xTiO2xe2x88x92yMnO, wherein x and y are % by mole and x and y satisfy the following conditions: 3.0xe2x89xa6xxe2x89xa69.0; and 0.1xe2x89xa6yxe2x89xa61.0, which method includes the steps of:
mixing a raw material containing Al, a raw material containing Ti and a raw material containing Mn;
molding the resulting mixture to yield a green compact; and
firing the green compact at a temperature not exceeding about 1310xc2x0 C.
Preferably, an alumina powder having a specific surface area of about 4 m2/g or more is used as the raw material containing Al, and a titanium dioxide powder having a specific surface area of about 3 m2/g is used as the raw material containing Ti in the above method. The alumina powder preferably has a specific surface area from about 4 to 5 m2/g. Additionally, the firing temperature is preferably about 1300xc2x0 C. or lower.